Aeolus is an ESA (European Space
Agency) Earth Explorer Core Mission -a science-oriented mission within
its Living Planet Program. The primary objective is to provide wind
profile measurements for an improved analysis of the global
three-dimensional wind field. The aim of the mission is to provide
global observations of wind profiles with a vertical resolution that
will satisfy the accuracy requirements of WMO (World Meteorological
Organization). Such knowledge is crucial to the understanding of the
atmospheric dynamics, including the global transport of energy, water,
aerosols, chemicals and other airborne materials - to be able to deal
with many aspects of climate research and climate and weather
prediction. ADM-Aeolus represents a demonstration project for the
Global Climate Observing System (GCOS). 1)2)3)4)5)6)7)8)9)10)11)12)13)

The measurement data will allow achievement of the primary goals of Aeolus:

- Provision of accurate wind
profiles throughout the troposphere and lower stratosphere eliminating
a major deficiency in the Global Observing System

- Direct contribution to the study of the Earth’s global energy budget

- Provision of data for the study of
the global atmospheric circulation and related features, such as
precipitation systems, the El Niño and the Southern Oscillation
phenomena and stratospheric/tropospheric exchange.

The secondary mission objectives are
related to the provision of data sets for model variation and
short-term “windclimatologies” allowing experts to:

- Validate climate models through the use of high quality wind profiles from a global measurement system

- Improve their understanding of
atmospheric dynamics and the global atmospheric transport and cycling
of energy, water, aerosols, chemicals and other airborne materials.

- Generate a number of derived products such as cloud top altitudes, aerosol properties and tropospheric height.

The ADM-Aeolus measurements will be
assimilated in numerical forecasting models, in order to enhance the
quality of operational short- and medium-range predictions. Expected
improvements are mainly due to the excellent horizontal and vertical
sampling capabilities of the instrument, combined with a continuous
availability of its data products within 3 hours after sensing.

Note: In the works of the Greek poet Homer, Aeolus
is the controller of the winds and ruler of the floating island of
Aeolia. In the Odyssey, he gave Odysseus a favorable wind and a bag in
which the unfavorable winds were confined. Odysseus' companions opened
the bag; the winds escaped and drove them back to the island. Although
he appears as a human in Homer, Aeolus later was described as a minor
god.

The ADM-Aeolus mission makes use of a single observation instrument, namely ALADIN
(Atmospheric Laser Doppler Instrument), employing the DWL (Doppler Wind
Lidar) measurement technique. The retrieval of wind speed relies on direct measurement
along the LOS (Line-of-Sight) by lidar using Doppler shift information
from atmospheric molecules and particles advected by wind. The ALADIN
observations will serve as input for NWP (Numerical Weather Prediction)
models. An extensive pre-development evaluation and assessment program
of ALADIN laser component technology was started in 2000.

ADM-Aeolus is
seen as a pre-operational mission, demonstrating new laser technology
and paving the way for future meteorological satellites to measure the
Earth’s wind.

Spacecraft:

Although the ADM-Aeolus satellite is
a new design, the platform is based on a heritage from other ESA
missions developed by Airbus DS (former EADS Astrium) including
CryoSat, and Rosetta. The aim has been to build a spacecraft that is
relatively simple to operate. This reduces the operating costs
throughout its lifetime, and is also important for the future since
similar Aeolus-type satellites are later envisaged for operational use.

The S/C structure, consisting of
aluminum honeycomb elements, uses a conventional box-shaped spacecraft
design (derived from Mars Express), upon which the observation
instrument is mounted via three isostatic bipods. The electronic boxes
of the bus and the associated satellite equipment are mounted on the
side panels.

Magnetometer: The magnetometer
(developed at LusoSpace, Portugal) of the ADM-Aeolus spacecraft employs
the AMR (Anisotropic Magneto Resistive) technology. The rationale for
using the AMR detector for the magnetometer development was due to
several advantages over fluxgate technology: 15)

- Detector production repeatability

- Lower cost

- Easier integration in a PCB (Printed Circuit Board)

- Possibility to generate external magnetic field in the chip by mean of built in coils.

The magnetometer is a small (credit
card surface dimension) and robust unit that can be used for several
LEO missions. Two flight models of the magnetometer will be flown on
ADM-Aeolus. In addition, a qualification model will fly on PROBA-2 as a
passenger to provide more flight heritage and in orbit data.

EPS (Electric Power Subsystem): Electric power is provided by two deployable solar wings of 14.5 m2
of total surface area. The triple-junction GaAs cells of the solar
arrays provide over 2.4 kW of power (with 1.4 kW of average power
required). The solar arrays are articulated toward the sun to optimize
their power output. Use of SADM (Solar Array Drive Mechanism) for
attitude regulation of the wings. The design includes a standard PCDU
(Power Control and Distribution Unit) responsible for solar array power
conditioning and distribution. A Li-ion battery of 64 Ah capacity is
being used for eclipse phases and LEOP (Launch and Early Orbit Phase). 16)

On-board autonomy: The
spacecraft is being designed to include a large amount of on-board
autonomy in all mission phases such that ground contact is needed no
more than once every 5 days even in the case of anomaly.

On-board data handling is performed
by an ERC-32 radiation tolerant processor with 6 MByte system RAM. The
subsystems are linked via MIL-STD-1533 data bus to the central
processor. A solid-state memory provides a capacity of 8 Gbit on-board
data storage.

Aeolus is conceived to allow simple
in-flight operation. The satellite has a five-day autonomy in case of
any single onboard failure, so that a single operator shift is
sufficient to monitor the satellite. In addition, the orbit has a seven
day repeat cycle, so that the complete operations timeline is repeated
on a weekly cycle, thus minimizing the effort for mission planning.

At the heart of
the avionics architecture are the CDMU (Command and Data Management
Unit) manufactured by RUAG, Sweden and the PCDU (Power Conversion and
Distribution Unit) manufactured by Patria, Finland. 17)18)

The SGM is a permanently powered
memory used to preserve data during PM reconfigurations and restarts.
Each PM has two software images stored in non-volatile memory, a
nominal mode image and a safe mode image. The RMs select which image to
download into RAM and execute.

Except for the AST (Autonomous Star
Tracker) subsystem, the CDMU is interfaced to all external units either
via discrete lines provided by the IO boards or via an external
MIL-STD-1553 bus. Each PM includes separate bus controllers allowing
the active PM to control both the ICB (Internal Control Bus) and the
external MIL-STD-1553 bus independently. The AST, manufactured by Terma
in Denmark, is interfaced directly to each PM via an RS422 HSUART (High
Speed Universal Asynchronous Receiver Transmitter).

The PCDU, which interfaces to the
CDMU via the external Mil-STD-1553 bus, provides regulated and
unregulated power outlets, shunt and battery charge control, solar
array deployment thermal knife control and individually switched heater
lines for thermal control. The power outlets supplying the TT&C
receivers and the reconfiguration units are non-switchable and are
protected by FCLs (Foldback Current Limiters). All other outlets are
switched and protected by LCLs (Latching Current Limiters). The shunt
regulation and battery charge control is fully implemented in the PCDU
electronics and requires no involvement from ground or the on-board
software under both nominal and failure conditions. Thermal knife
drivers and deployment micro-switch status acquisition and conditioning
are provided to support solar array deployment.

On-board autonomy architecture: One
of the simplest methods to achieve on-board autonomy is to implement an
on-board schedule that is loaded fully under ground responsibility.
Such an autonomy approach is straight forward to test and validate
since only basic functionalities such as command insertion, command
deletion and command execution at scheduled time have to be tested. In
particular there is no need to develop and test any logic relating one
command to another and there is no need to develop and test any logic
for selecting which commands to schedule. This was the approach adopted
for ADM-Aeolus with two simple schedules being implemented, one based
on time and the other based on orbit position.

Although this
approach works well under nominal circumstances, it is not tolerant to
failures that occur in the system such that, by the time the commands
are due for execution, they are no longer valid or allowed. In
particular such a system design approach is vulnerable to the
following:

1) The scheduled commands address a physical unit that has failed and has been replaced by its redundant unit.

2) A scheduled command fails to execute successfully because a reconfiguration is occurring.

3) Commands to one unit are only
allowable if another unit or subsystem is in a particular state and
must not be executed if this condition is not met.

4) Scheduled commands are part of a
functional sequence of commands and so are dependent on the successful
execution of previous scheduled commands.

5) Complex critical operations,
such as solar arrays deployment, require the execution of decision
branches and must be executed even if the CDMU is reconfigured or
restarted.

During the design stage the
potential vulnerability of the AEOLUS scheduled operations to the above
cases was assessed and the solutions taken to avoid them (Ref. 17).

FDIR (Failure Detection, Isolation and Recovery):

The overall FDIR concept adopted in
Aeolus is driven by the objective to minimize ground intervention both
during nominal operations and in failure scenarios.

The autonomous multi-layer FDIR architecture must include monitors to identify all failures that:

- Directly endanger the unit itself
or risk propagation to other units as identified in the Satellite and
lower level FMECAs (Failure Modes and Effects Criticality Analysis)

- Corrupt or significantly degrade
functions necessary for the correct functioning of the spacecraft in
the current spacecraft mode / configuration [these failures may be
identified in the FMECAs and HSIAs (Hardware Software Interaction
Analysis) or may be “feared events”]

- Corrupt or significantly degrade functions necessary for data dissemination to the ground.

A high speed FDIR MIL-STD-1553 bus
was established to monitor bus protocol status messages to identify a
loss of communication and allow start of recovery within 1 second. For
each unit, feared events are identified based on the function of the
unit in the overall design and also based on the satellite and unit
FMECA and HSIA documents (Ref. 17).

The Aeolus FDIR concept is built around top-down onboard control architecture: (Ref. 12)

• At the highest level hot
redundant TTR(TM, TC and Reconfiguration) boards within the CDMU
contain Reconfiguration Modules which oversee the health and function
of the CDMU and flight software by monitoring hardware alarm inputs and
performing CDMU resets, reconfigurations and switches to Safe Mode as
appropriate.

• At the next level the CDMU
application software monitors and controls the spacecraft units by
monitoring on board parameters and autonomously sending control
commands in response to parameter out of range events.

• At the lowest level some
units perform their own built-in health checks and report this through
the TM to the CDMU software.

For the platform functions, the FDIR
needs to ensure that the spacecraft can safely recover from single
level failures either by resuming operations autonomously or by
switching to predefined redundant configurations. For ALADIN, the FDIR
needs to ensure instrument safety by both stopping scheduled operations
and switching the instrument into a safe and stable configuration or by
switching ALADIN into Survival mode.

Redundancy princple: In case of
on-board failure detection during any of the mission phases, the
on-board control system will attempt to recover operational status by
switching to redundant units. In order to avoid the loss of platform
functions mandatory for the mission, the redundancy concept has to be
such that a single failure does not cause permanent loss of essential
platform functions. All units have to therefore be independent of their
redundant alternatives. This includes provisions to prevent malfunction
or elimination of redundant units by a common cause.

The S/C mass at launch is about 1360
kg of which 266 kg are allocated to the payload. Its size is 1.74 m x
1.9 m x 2.0 m in launch configuration, limited by the payload envelop.
The solar arrays of 13 m span have three panels on each side. The
design life is 3 years. The prime spacecraft contractor is EADS Astrium
Ltd., Stevenage, UK (contract award in Oct. 2003). Further Astrium
sites in Germany and France are involved in the spacecraft development.
19)20)21)22)

RF communications: TT&C
communications are based on standard S-band links, the uplink data rate
is 2 kbit/s the downlink data rate is up to 8 kbit/s. The measurement
data are dumped via an X-band transmitter with 10 Mbit/s data rate. S/C
operations are performed at ESOC (Darmstadt, Germany) using the Kiruna
TT&C station. - The measurement data are received nominally by the
ground station in Svalbard (Spitzbergen). Additional X-band receiving
stations (antenna diameter as small as 2.4 m) can easily be added to
provide a shorter data delivery time.

Launch: The Aeolus spacecraft
was launched on 22 August 2018 (21:20 GMT) on a Vega vehicle,
designated VV12, from Kourou,French Guiana. Some 55 minutes later,
Vega’s upper stage delivered Aeolus into orbit and contact was
established through the Troll ground station in Antarctica. The
satellite is being controlled from ESA’s ESOC (European Space
Operations Center) in Darmstadt, Germany. Controllers will spend the
next few months carefully checking and calibrating the mission as part
of its commissioning phase. 23)24)25)

Figure 5: Aeolus heads for orbit (image credit: ESA/CNES/Arianespace)

On Sept. 7, 2016, ESA and
Arianespace signed a contract to secure the launch of the Aeolus
satellite. With this milestone, a better understanding of Earth’s
winds is another step closer. With the main technical hurdles resolved
and the launch contract now in place.

In 2015, a launch of ADM-Aeolus is
expected in 2017 (the original launch date was 2007, then in 2011). The
launch vehicle is Vega and the launch site is Kourou. 26)27)

Baseline change in the autumn of 2010: Change from burst mode to “continuous mode” operation.
An in-depth review of the ALADIN laser, involving independent laser
experts, identified the need to make some substantial modifications to
the current design in order to regain adequate performance margins for
the three years of in-orbit operation. The most significant
modification is a change of the operational principle, from
‘burst’ to ‘continuous’ mode. A requirement
review was completed to define the associated requirements mode.

Stable and complete versions of
the end-to-end simulator and ground payload data processing software
are available, but they need to be upgraded to support the new
continuous mode of the ALADIN instrument. These significant changes to
the instrument design have delayed the planned launch date to mid-2013.
28)29)30)

• September 3, 2019: For
the first time, ESA has performed a 'collision avoidance maneuver' to
protect one of its spacecraft from colliding with a satellite in a
large constellation.

• Constellations are
fleets of hundreds up to thousands of spacecraft working together in
orbit. They are expected to become a defining part of Earth’s
space environment in the next few years.

• As the number of
satellites in space dramatically increases, close approaches between
two operated spacecraft will occur more frequently. Compared with such
'conjunctions' with space debris – non-functional objects
including dead satellites and fragments from past collisions –
these require coordination efforts, to avoid conflicting actions.

• Today, the avoidance
process between two operational satellites is largely manual and ad hoc
– and will no longer be practical as the number of alerts rises
with the increase in spaceflight.

• “This example
shows that in the absence of traffic rules and communication protocols,
collision avoidance depends entirely on the pragmatism of the operators
involved,” explains Holger Krag, Head of Space Safety at ESA.
— “Today, this negotiation is done through exchanging
emails - an archaic process that is no longer viable as increasing
numbers of satellites in space mean more space traffic.”

• ESA is proposing an
automated risk estimation and mitigation initiative as part of its
space safety activities. This will provide and demonstrate the types of
technology needed to automate the collision avoidance process, allowing
machine generated, coordinated and conflict-free maneuver decisions to
speed up the entire process – something desperately needed to
protect vital space infrastructure in the years to come.

- Data is constantly being issued by
the 18th Space Control Squadron of the US Air Force, who monitor
objects orbiting in Earth’s skies, providing information to
operators about any potential close approach.

- With this data, ESA and others are
able to calculate the probability of collision between their spacecraft
and all other artificial objects in orbit.

- About a week ago, the US data
suggested a potential ‘conjunction’ at 11:02 UTC on Monday,
2 September, between ESA’s Aeolus satellite
and Starlink44 – one of the first 60 satellites recently launched
in SpaceX’s mega constellation, planned to be a 12 000 strong
fleet by mid-2020.

- As days passed, the probability of
collision continued to increase, and by Wednesday 28 August the team
decided to reach out to Starlink to discuss their options. Within a
day, the Starlink team informed ESA that they had no plan to take
action at this point.

- ESA’s threshold for
conducting an avoidance maneuver is a collision probability of more
than 1 in 10 000, which was reached for the first time on Thursday
evening (29 August).

- An avoidance
maneuver was prepared which would increase Aeolus’ altitude by
350 m, ensuring it would comfortably pass over the other satellite, and
the team continued to monitor the situation.

- On Sunday (1 September), as the
probability continued to increase, the final decision was made to
implement the maneuver, and the commands were sent to the spacecraft
from ESA’s mission control center in Darmstadt, Germany.

- At this moment, chances of collision were around 1 in 1000, 10 times higher than the threshold.

- On Monday morning (2 September),
the commands triggered a series of thruster burns at 10:14, 10:17 and
10:18 UTC, half an orbit before the potential collision.

- About half an hour after the
conjunction was predicted, Aeolus contacted home as expected. This was
the first reassurance that the maneuver was correctly executed and the
satellite was OK.

- Since then, teams on the ground
have continued to receive scientific data from the spacecraft, meaning
operations are back to normal science-gathering mode.

- Contact with Starlink early in the
process allowed ESA to take conflict-free action later, knowing the
second spacecraft would remain where models expected it to be.

New space

- Since the first satellite launch
in 1957, more than 5500 launches have lifted over 9000 satellites into
space. Of these, only about 2000 are currently functioning, which
explains why 90% of ESA’s avoidance maneuvers are the result of
derelict and uncontrollable ‘space debris’.

- In the years to come,
constellations of thousands of satellites are set to change the space
environment, vastly increasing the number of active, operational
spacecraft in orbit.

- This new technology brings
enormous benefits to people on Earth, including global internet access
and precise location services, but constellations also bring with them
challenges in creating a safe and sustainable space environment.

Space rules

- “No one was at fault here,
but this example does show the urgent need for proper space traffic
management, with clear communication protocols and more
automation,” explains Holger.

- “This is how air traffic
control has worked for many decades, and now space operators need to
get together to define automated maneuver coordination.”

Autonomous spaceflight

- As the number of satellites in
orbit rapidly increases, today's 'manual' collision avoidance process
will become impossible, and automated systems are becoming necessary to
protect our space infrastructure.

- Collision avoidance maneuvers take
a lot of time to prepare – from determining the future orbital
positions of functioning spacecraft, to calculating the risk of
collision and the many possible outcomes of different actions.

- ESA is preparing to automate this
process using artificial intelligence, speeding up the processes of
data crunching and risk analysis, from the initial warning of a
potential conjunction to the satellite finally moving out of the way.

- Such use of space-based communication links can save precious time when sending maneuver commands at the last minute.

- Under its Space Safety
activities, ESA plans to invest in technologies required to
automatically process collision warnings, coordinate maneuvers with
other operators and send the commands to spacecraft entirely
automatically, ensuring the benefits of space can continue to be
enjoyed for generations to come.

• July 23, 2019: ESA’s
Aeolus satellite, which carries the world’s first space Doppler
wind lidar, has been delivering high-quality global measurements of
Earth’s wind since it was launched almost a year ago. However,
part of the instrument, the laser transmitter, has been slowly losing
energy. As a result, ESA decided to switch over to the
instrument’s second laser – and the mission is now back on
top form. 32)

Figure 7: Shortly after switching
over from the first to the second laser, Aeolus is delivering
high-quality measurements of Earth’s wind. Currently, instrument
and data processing refinements are ongoing, which will enhance the
data product quality even more in the coming weeks. The figure shows
measurements by Aeolus while crossing the African continent between
Turkey (on the right) and the Southern Ocean (left). Aeolus measures
winds from the surface up to about 25 km altitude. Strong easterly
winds are visible around the tropopause at 15 km altitude over north
Africa (green, yellow and orange), and the strong westerly winds (blue
and purple colors) in the upper troposphere and lower stratosphere as
the satellite moves into the area of the ‘roaring forties’
over the Southern Ocean. Thick clouds block the laser signal and hence
prevent measurements to be taken within or below the clouds (white
areas between 0 and 10 km altitude), image credit: ESA

- Developing novel space technology
is always a challenge, and despite the multitude of tests that are done
in the development and build phases, engineers can never be absolutely
certain that it will work in the environment of space.

- Aeolus is, without doubt, a
pioneering satellite mission – it carries the first instrument of
its kind and uses a completely new approach to measuring wind from
space.

- The instrument, called Aladin, not
only comprises the laser transmitters, but also one of the largest
telescopes ESA has put into orbit and very sensitive receivers that
measure the minute shifts in wavelength of light generated by the
movement of molecules and particles in the atmosphere caused by the
wind.

Figure 8: The
state-of-the-art Aladin instrument incorporates two powerful lasers, a
large telescope and very sensitive receivers. The laser generates
ultraviolet light that is beamed towards Earth. This light bounces off
air molecules and small particles such as dust, ice and droplets of
water in the atmosphere. The fraction of light that is scattered back
towards the satellite is collected by Aladin’s telescope and
measured (image credit: ESA)

- Aladin, works by emitting short,
powerful pulses of ultraviolet light from a laser and measures the
Doppler shift from the very small amount of light that is scattered
back to the instrument from these molecules and particles to deliver
vertical profiles that show the speed of the world’s winds in the
lowermost 30 km of the atmosphere.

- While scientists and meteorology
centers have been thrilled with the data produced by Aeolus, the first
laser’s energy was becoming a concern – and in June, energy
levels dipped to the point that the quality of the wind data was set to
be compromised.

- Tommaso Parrinello, ESA’s
Aeolus mission manager, said, “With the power from the first
laser declining, we decided to turn it off and activate the second
laser, which the instrument was equipped with to ensure we could
address an issue such as this.

- “Switching to the second
laser appears to have done the trick so we’re back in business.
And, we are confident that the instrument will remain in good shape for
years to come.”

Figure 9: This photo, which was
taken in the cleanroom when Aeolus was being built, shows the
instrument’s two lasers. They are the two large square plate-like
items in the middle. Aeolus carries the world’s first space
Doppler wind lidar. It works by emitting short, powerful pulses of
ultraviolet light from a laser and measures the Doppler shift from the
very small amount of light that is scattered back to the instrument
from molecules and particles in the atmosphere to deliver vertical
profiles that show the speed of the world’s winds in the
lowermost 30 km of the atmosphere (image credit: Airbus Defence and
Space)

- Denny Wernham, ESA’s Aeolus
instrument manager, added, “The great news is that the second
laser’s energy is, so far, very stable, which is what we expected
since this laser is actually better than the first. This is because we
have more scope to adjust it in orbit to retain the performance needed.

- “I would like to stress that
despite the first laser’s drop in energy, it worked for nearly a
year and provided a vital dataset for our stakeholders. It accumulated
nearly one billion shots, which is a record for a high-power
ultraviolet laser in space, and we can always go back to it if we need
to later in the mission.”

- The ECMWF (European Center for Medium Range Weather Forecasting) is also enthusiastic about the data now being delivered.

- Michael Rennie at ECMWF, said,
“We were very happy to see the wind data after the switch, and
given the fact that when Aeolus was using its first laser we could see
that it can improve our weather forecasts off-line, we are expecting
even better results with the new setup.

- “Towards the end of the year, we hope that we will be feeding data from Aeolus into our forecasts in real time.”

- “We are very much looking
forward to seeing several weather-forecast impact assessments by
European, American and Asian meteorological centers at a meeting with
our community in September 2019.

- “These assessments compare
the impact of Aeolus with the impact of measurements by other weather
satellites and observations in the World Meteorological Organization
Global Observing System.

- “Towards the end of 2019,
further scientific studies will also start using Aeolus wind
observations to learn more about the role of winds in the
atmosphere–land–ocean system and how small and large-scale
winds will alter as our climate changes.”

• April 5, 2019: Assessing the
accuracy of data being returned by completely new technology in space
is a challenging task. But this is exactly what engineers and
scientists have been dedicating their time to over the last months so
that measurements of the world’s winds being gathered by Aeolus
can be fed confidently into weather forecast models. 33)

- Carrying breakthrough laser
technology, the Aeolus satellite – an ESA Earth Explorer mission
– was launched in August 2018. Its novel Aladin instrument, which
comprises a powerful laser, a large telescope and a very sensitive
receiver, measures the wind by emitting short, powerful pulses of
ultraviolet light down into the atmosphere.

- It is the first satellite mission
to provide profiles of Earth’s wind globally. Its near-realtime
observations will soon be made available to weather forecasters around
the world. These observations are set to improve the accuracy of
weather forecasts as well as advance our understanding of atmospheric
dynamics and processes linked to climate variability.

- Before ESA can declare that the
data good enough to be included in forecasts, the data have to be
carefully calibrated and validated. Part of this process has involved
gathering measurements of wind, aerosols and clouds from the ground,
aircraft and from other satellites to compare them with measurements
being delivered by Aeolus.

- Also, in preparation for ingesting
the data into their forecasts, a number of weather forecasting centers
around the world have started to compare the Aeolus winds with their
models.

- So, after several months of
calibration and validation exercises, around 100 scientists and
engineers from universities, research institutes and weather centers in
Europe, the US, Canada, Japan and China gathered recently at
ESA’s center of Earth observation in Frascati, Italy to review
the latest results from the Aeolus data investigations.

Figure 10: The image shows winds
measured by Aeolus over western Europe on 10 March 2019. Red indicates
wind blowing from east to west (easterlies) and blue indicates wind
blowing from west to east (westerlies). The strong westerly wind in the
jet stream, with speeds of more than 200 km/hr, is clearly visible at
the altitude of around 10 km. On this day, very strong winds extended
from the jet stream all the way down to the surface and caused problems
for traffic and construction, for example. Black areas indicate where
the satellite could not measure winds owing to thick cloud layers
(image credit: ECMWF–M. Rennie)

The European Center for
Medium-range Weather Forecast (ECMWF) and the German Weather Service
(DWD) preliminary analyses showed that Aeolus winds are improving
forecasts, particularly in the troposphere, which is the part of the
atmosphere between the ground and about 16 km high.

Lars Isaksen, principal scientist
at ECMWF, said, “Aeolus’ Aladin is the only instrument that
provides wind profiles from space. Wind profiles, especially over
remote areas, are very important for numerical weather prediction.
ECMWF is heavily involved in processing, calibrating and validating the
Aeolus wind data, and in just seven months after the satellite was
launched, we and other weather centers have carried out numerous impact
studies. These results are very promising and indicate that Aeolus
winds will improve weather forecasts and help us better understand
global wind circulation.”

Examples of
results presented at the workshop included the storm that hit the UK
and parts of Europe on 10 March and Cyclone Idai that devastated
Mozambique, Malawi and Zimbabwe.

Figure 11: Wind measured by the
Aeolus satellite while crossing the Cyclone Idai west of Madagascar on
11 March 2019. Red indicates wind blowing from east to west
(easterlies) and blue indicates wind blowing from west to east
(westerlies). Since Aeolus measures wind in the cloud-free atmosphere,
and within thin clouds and on top of thick clouds, the measurements
here are those surrounding Idai. The black patch is the part of the
cyclone, which was covered by a thick cover of spiral-shaped clouds.
The image shows strong easterly winds north of the hurricane (in red on
the left of the image), with wind speeds up to 150 km/hr (above 40
m/s). In the upper right corner (altitude of 22–25 km), the
tropical stratospheric easterly jet can be seen in red, and lower down
on the right (altitude of 10–16 km) the sub-tropical westerly jet
in the southern hemisphere is visible in blue (image credit:
ECMWF–M. Rennie)

Figure 12: Cyclone Idai west of
Madagascar. Captured by the Copernicus Sentinel-3 mission, this image
shows Cyclone Idai on 13 March 2019 west of Madagascar and heading for
Mozambique. Here, the width of the storm is around 800–1000 km,
but does not include the whole extent of Idai. The storm went on to
cause widespread destruction in Mozambique, Malawi and Zimbabwe. With
thousands of people losing their lives, and houses, roads and croplands
submerged, the International Charter Space and Major Disasters and the
Copernicus Emergency Mapping Service were triggered to supply maps of
flooded areas based on satellite data to help emergency response
efforts (image credit: ESA, the image contains modified Copernicus
Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)

- The value of having different
satellite instruments observing the same weather event is important for
gathering as much information as possible to improve the accuracy of
weather forecasts and so that people affected by severe weather can
take necessary action.

- Tommaso Parrinello, ESA’s
Aeolus Mission Manager, said, “We are really happy with the data
Aeolus is returning. We also see how the mission can add complementary
information to satellites carrying optical instruments such as the
Copernicus Sentinel-3 and the satellites carrying radar such as the
Copernicus Sentinel-1. While comparisons with ground-based
instrumentation and weather models are currently ongoing to refine the
calibration and data processing, we expect that the quality of the
Aeolus data will be high enough around the end of this year –
after which the data will be ready for scientific research and for
weather forecasting.”

• February
11, 2019: Since launch, engineers and scientists have been carefully
checking the information that this pioneering mission is delivering on
the world’s winds – and now it’s time for the next
phase. Although our daily weather forecasts are pretty reliable, they
still need to be improved further and to do this meteorologists
urgently need direct measurements of the wind. 34)

- However, this is no easy task as extraordinary technology is needed to measure the wind from space.

- Nevertheless, ESA’s Aeolus
satellite has been designed to do just this. It carries the first
instrument of its kind and uses a completely new approach to measuring
wind.

- Since this is such novel and
challenging technology, scientists and engineers have had their work
cut out assessing how the satellite is functioning in orbit and
checking the quality of the data it is returning.

- For example, they have been
comparing this new data with modelled data at the ECMWF (European
Center for Medium-Range Weather Forecasts) and have already established
improvements to the forecast model thanks to the additional data from
Aeolus.

- This will have a positive impact on weather forecast accuracy in general.

- ESA’s Aeolus project
manager, Anders Elfving, said, “This satellite mission is
certainly a challenging one, but I’m very happy to say that we
are now formally out of the commissioning phase, which encompasses the
first four months of a mission’s life in orbit when we do all the
checks and tweaks.”

- “We still have some work to
do to make sure Aeolus delivers on its promise as we have to improve on
the way the data is processed taking into account the peculiarities of
its instrument. And, we must remember that this is a completely new
type of mission, so we are learning all the time. We also have field
campaigns going on all over the world to help with the process of
calibration and validation. This means measurements of the wind are
being taken from the ground, from balloons and from aircraft to compare
with measurements we are getting from space.- At this stage, the
results are expected to be announced in March.”

- One recent field campaign has been
carried out in Germany by DLR (German Aerospace Center). This involved
flying an aircraft directly under Aeolus’ orbital path and taking
more or less simultaneous measurements with an airborne version of the
satellite instrument.

Figure 13: Comparing wind
measurements: As part of the working being done to calibrate and
validate measurements from ESA’s Aeolus wind satellite,
scientists have been taking similar measurements from an aircraft
carrying an airborne version of the satellite instrument. instrument.
The pilot flies the plane under the satellite as it orbits above so
that measurements of wind can be compared (image credit: ESA/DLR)

• February 7, 2019: Following
the launch of Aeolus on 22 August 2018, scientists have been busy
fine-tuning and calibrating this latest Earth Explorer satellite.
Aeolus carries a revolutionary instrument, which comprises a powerful
laser, a large telescope and a very sensitive receiver. It works by
emitting short, powerful pulses –50 pulses per second –of
ultraviolet light from a laser down into the atmosphere. The instrument
then measures the backscattered signals from air molecules, dust
particles and water droplets to provide vertical profiles that show the
speed of the world’s winds in the lowermost 30 km of the
atmosphere. These measurements are needed to improve weather forecasts.
As part of the working being done to calibrate this novel mission,
scientists have been taking similar measurements from an aircraft
carrying an airborne version of Aeolus’ instrument. The pilot
flies the plane under the satellite as it orbits above so that
measurements of wind can be compared. 35)

Figure 14: Flying under Aeolus (video credit: ESA)

• September 12, 2018: Just one
week after ESA’s Aeolus satellite shone a light on our atmosphere
and returned a taster of what’s in store, this ground-breaking
mission has again exceeded all expectations by delivering its first
data on wind – a truly remarkable feat so early in its life in
space. 36)

- Florence Rabier, Director General
of the ECMWF (European Centre for Medium-Range Weather Forecasts),
said, “We always knew that Aeolus would be an exceptional
mission, but these first results have really impressed us. The
satellite hasn’t even been in orbit a month yet, but the results
so far look extremely promising, far better than anyone expected at
this early stage. We are very proud to be part of the mission. Aeolus
looks set to provide some of the most substantial improvements to our
weather forecasts that we’ve seen over the past decade.”

- ESA’s Aeolus mission
scientist, Anne Grete Straume, explained, “These first wind data
shown in the plot made by ECMWF are from one orbit. In the profile we
can see large-scale easterly and westerly winds between Earth’s
surface and the lower stratosphere, including jet streams. In
particular, you can see strong winds, called the Stratospheric Polar
Vortex, around the South Pole. These winds play an important role in
the depletion of the ozone layer over the South Pole at this time of
the year.”

- Named after Aeolus, who in Greek
mythology was appointed ‘keeper of the winds’ by the Gods,
this novel mission is the fifth in the family of ESA’s Earth
Explorers, which address the most urgent Earth-science questions of our
time.

- It carries the first instrument of its kind and uses a completely new approach to measuring the wind from space.

- ESA’s Earth Explorer Program
manager, Danilo Muzi, said, “Aeolus carries revolutionary laser
technology to address one of the major deficits in the Global Observing
System: the lack of direct global wind measurements. The essence of an
Earth Explorer mission is to deliver data that advances our
understanding of our home planet and that demonstrates cutting-edge
space technology. With the first light measurements and now these
amazing wind data, Aeolus has wowed us on both fronts.”

Figure 15: First wind data from
ESA’s Aeolus satellite. These data are from three quarters of one
orbit around Earth. The image shows large-scale easterly and westerly
winds between Earth’s surface and the lower stratosphere,
including jet streams. As the satellite orbits from the Arctic towards
the Antarctic, it senses, for example, strong westerly winds streams,
called tropospheric vortices (shown in blue) each side of the equator
at mid latitudes. Orbiting further towards the Antarctic, Aeolus senses
the strong westerly winds (shown in blue left of Antarctica and in red
right of Antarctica) circling the Antarctic continent in the
troposphere and stratosphere (Stratospheric Polar Vortex). The overall
direction of the wind is the same along the polar vortex, but because
the Aeolus wind product is related to the viewing direction of the
satellite, the color changes from blue to red as the satellite passes
the Antarctic continent (image credit: ESA/ECMWF)

Figure 16:
Ozone hole over Antarctica on 4 September 2018. Strong winds, called
the Stratospheric Polar Vortex, around the South Pole play an important
role in the depletion the ozone at this time of the year. Low ozone is
shown in blue and high in pink (image credit: KNMI–Temis,
released on 12 September 2018)

• September 5, 2018: The
ALADIN instrument on Aeolus has been turned on and is now emitting
pulses of ultraviolet light from its laser, which is fundamental to
measuring Earth’s wind. And, this remarkable mission has also
already returned a tantalizing glimpse of the data it will provide. 37)

- Aeolus carries a revolutionary
instrument, which comprises a powerful laser, a large telescope and a
very sensitive receiver. It works by emitting short, powerful pulses
– 50 pulses per second – of ultraviolet light from a laser
down into the atmosphere. The instrument then measures the
backscattered signals from air molecules, dust particles and water
droplets to provide vertical profiles that show the speed of the
world’s winds in the lowermost 30 km of the atmosphere.

- The mission is now being
commissioned for service – a phase that lasts about three months.
One of the first things on the ‘to do’ list was arguably
the one of the most important: turn on the instrument and check that
the laser works.

- ESA’s Director of Earth
Observation Programs, Josef Aschbacher, explained, “Aeolus is a
world premiere. After the launch two weeks ago the whole community has
been anxiously awaiting the switch-on of the ultra-violet laser, which
is a real technological marvel. This has been successful. We have
pioneered new technology for one of the largest data gaps in
meteorology – global wind profiles in cloud-free atmosphere. I am
grateful to all who have made this success possible.”

- ESA’s Aeolus project
manager, Anders Elfving, added, “Aeolus has been one of the most
challenging missions on ESA’s books. And, unsurprisingly, we have
had to overcome a number of technical challenges. After many years in
development, we had absolute confidence that it would work in space,
but it was still somewhat nerve-racking when we turned on the
instrument a few days ago. But the years of work certainly appear to
have paid off. After turning it on, we started slowly and steadily
increasing the power. It is now emitting at high power – and we
couldn’t be happier.”

- Richard Wimmer from Airbus Defence
and Space noted, “It is a very exciting time to have Aeolus
safely in orbit and doing what we and our industrial teams spent years
building it to do.”

- Michael Rennie from the ECMWF
(European Centre for Medium-Range Weather Forecasts), added, “At
this very early stage in the mission – just three days after the
instrument was switched on – Aeolus has already exceeded
expectations by delivering data that show clear features of the
wind.”

- With Aeolus instrument healthy and
performing well, engineers will continue ticking off other items on the
‘commissioning to do list’ so that in a few months Aeolus
will be ready to deliver essential information to improve our knowledge
of atmospheric dynamics, further climate research and improve weather
forecasts.

Figure 17: First light from
Aeolus. Following the launch of Aeolus on 22 August, this extraordinary
satellite is not only emitting pulses of ultraviolet light from its
laser, but has also measured light backscattered from air molecules and
cloud tops. The measurements show a full orbit around Earth, from the
Arctic to the Antarctic, and back. For calibration purposes the signal
backscattered from Earth’s surface is used, which is also seen in
these results (image credit: ESA) .

• August 24, 2018: Having
worked around the clock since the launch of Aeolus on 22 August, teams
at ESA’s control center in Germany have declared today that the
critical first phase for Europe’s wind mission is complete. 38)

- Once in orbit, Aeolus separated
from the Vega launcher and began its free-flying journey, unfolding its
solar arrays, turning its radio antenna toward Earth and sending
signals to ground stations in Australia and Antarctica to signify that
all is well.

- An initial radio signal from Aeolus was picked up at 00:15 CEST on 23 August by a special launcher tracking dish, dubbed NNO-2, at ESA’s New Norcia station in Australia — the newest in the Agency’s network of communication antennas.

- This first,
simple, ‘hello’ was followed just 15 minutes later by the
official data link that was established at the Norwegian Troll Satellite Station
in Antarctica. With this full data link, mission teams at ESOC became
able to send commands to the satellite and receive the data it will go
on to collect.

- Flight control teams guided the
satellite through this tense period, working to ensure Aeolus was
safely configured and ready for its next milestone: in-orbit
commissioning.

- During the commissioning phase of
a satellite, controllers nudge it slightly to optimize its position in
orbit, and perform tests to ensure the health of its instruments. This
step is unique for every satellite, and for Aeolus it is expected to
last for several months.

- The main commissioning objective
of Aeolus is to fully check out, calibrate and understand the behavior
of all systems onboard the spacecraft, now that has taken up its new
residence in space. The absolute centerpiece of this, ESA’s
newest satellite, will be the switch-on and first light of the
hypermodern Aladin lidar instrument.

- Once this is done, the real
challenge will be to fully calibrate, characterize and tune the
instrument, finally making it able to get to work measuring
Earth’s winds.

The DWL (Doppler Wind Lidar) operation principle of ALADIN:

DWL is an active observation
technique; the instrument fires laser pulses towards the atmosphere and
measures the resulting Doppler shift of the return signal,
backscattered at different levels in the atmosphere. The frequency
shift results from the relative motion of the scatter elements along
the sensor line of sight. This motion relates to the mean wind in the
observed volume (cell). The measurement volume is determined by the
ground integration length of 50 km (sample size), the required height
resolution and the width of the laser footprint. The measurements are
repeated at intervals of 200 km.

Light is scattered either by
interaction with aerosol or cloud particles (Mie scattering) or by
interaction with air molecules (Rayleigh scattering). The two
scattering mechanisms exhibit different spectral properties and
different wavelength dependencies such that instruments evaluating only
one signal type or both in separate processing chains can be
constructed.

To improve the detection of the
Rayleigh signal, the laser emits light pulses in the UV spectral region
(355 nm). Detection of the backscatter light and analysis of the
Doppler shift is done with high-resolution spectrometers (about 5 x 108 resolving power).

For lidar techniques where the shape
of the backscattered light cannot be directly measured in detail, it is
important to know what shape is expected in order to calculate the
speed, abundance, temperature or chemical composition of molecules in
the atmosphere. - The shape of the backscattered light is described by
‘Rayleigh-Brillouin scattering theory’, where the Rayleigh
scattering is related to the temperature and Brillouin scattering is
related to pressure fluctuations in the atmosphere. The shape of the
Rayleigh-Brillouin backscattered light is described by the
‘Tenti’ model, which was created in the early 1970s. This
model is used worldwide to interpret atmospheric lidar measurements. 39)

Although early
ESA studies showed this model to be suitable for interpreting data from
the Agency’s satellites carrying lidars, it was decided to launch
a new laboratory experiment, through ESA’s General Studies
Program, to see if there was still room for improvement. An advanced
model would lead to even better accuracy in lidar measurements.

The study was led by Wim Ubachs of
the Laser Centre at the VU University Amsterdam in the Netherlands. The
team included participants from the VU University Amsterdam, the
University of Nijmegen, Eindhoven University of Technology, the KNMI
(Royal Netherlands Meteorological Institute) and the German Aerospace
Center, DLR. 40)

Figure 19:
Example of Rayleigh-Brillouin scattering of light emitted at a
wavelength (green line) as a function of its intensity (I), at a
pressure of one atmosphere (image credit: ESA)

Legend to Figure 19:
The red line shows the emitted light after Rayleigh scattering by
molecules. The blue line shows the light after both Rayleigh and
Brillouin scattering.

Measurements of Rayleigh-Brillouin
scattering were taken for a range of pressures and gases,
representative of Earth’s atmosphere. The measurements were
compared to the Tenti model, and as a result the model could be
improved. The experiment concluded that the updated Tenti model now
describes the shape of the backscattered light from nitrogen and oxygen
to within an accuracy of 98%. It was also confirmed that atmospheric
water vapor does not affect the Rayleigh-Brillouin line shape. In
addition, the scattering profiles from nitrogen, oxygen and air were
shown to be the most accurate ever measured worldwide and will now form
the basis for further scientific research into Rayleigh-Brillouin
scattering.

The study has delivered a wide
variety of profiles that are important, not only to ESA’s lidar
missions, but also to other scientists working with lidar instruments.
Some important issues dealing with the understanding of the profiles
related to wavelength, scattering angle and temperature dependencies
and polarization effects are still open and will be further studied in
a follow-on activity with ESA.

Observation configuration:

The satellite is flown with the
ALADIN instrument pointing toward Earth in a plane quasi-perpendicular
to the flight path and 35º offset from nadir in the anti-sun
direction. The measurement geometry is depicted in Figure 20.
The LOS is oriented such that the relative velocity at the intersection
with the Earth is zero (yaw steering). All measurements are taken along
the LOS. The Doppler shift of the backscatter signal reflects the
relative wind speed along the LOS and has to be processed to a
horizontal wind speed component, HLOS (Horizontal Line-of-Sight), referenced to the ground.

The measurement volume of the return
signal from a single shot is defined by the lateral extension of the
transmitted beam (a few meters in diameter) and the time gating of the
receiver, which is adapted to the desired vertical resolution (250 m to
2 km or more). Due to the fact that the signal from a single shot is
too weak for the evaluation, 700 shots along a ground measurement track
of 87 km have to be accumulated and integrated.

Measurement profile: The onboard
instrument is operated at a duty cycle of 25% to obtain wind profile
separation. An active operation cycle lasts 7 seconds (equivalent to
about 87 km ground track), followed by a gap in observations of 21
seconds (equivalent of nearly 150 km ground track). Winds can be
measured in clear air (i.e., above or in the absence of thick clouds),
and within and through thin clouds (e.g., cirrus).

Sensor complement: (ALADIN)

ALADIN (Atmospheric Laser Doppler Instrument):

The instrument is being developed by
Airbus DS (former EADS Astrium SAS), Toulouse, France as prime
contractor of an industrial consortium. ALADIN is an incoherent direct
detection lidar incorporating a fringe-imaging receiver (analyzing
aerosol and cloud backscatter) and a double-edge receiver (analyzing
molecular backscatter). The lidar emits laser pulses towards the
atmosphere, then acquires, samples, and retrieves the frequency of the
backscattered signal. The overall ALADIN instrument architecture is
based on a 60 mJ diode-pumped frequency-tripled Nd:YAG laser operating
in the ultraviolet (solid-state laser technology). The instrument
consists of three major elements: a transmitter, a combined Mie and
Rayleigh backscattering receiver assembly, and the opto-mechanical
subsystem (a telescope with a 1.5 diameter). After integration, the
telescope wavefront error has been measured within the specification
(better than half a wavelength). This is a key parameter for minimizing
the bias error on the wind speed. 41)42)43)44)45)46)47)48)49)50)51)52)53)54)55)56)

The power laser is composed of a low
power oscillator (10 mJ output energy) and two power amplifiers to
generate light pulses with 150 mJ energy at the fundamental wavelength
of Nd:YAG (1064 nm). This is converted to 60 mJ pulses in the UV (355
nm) by a frequency tripler. The oscillator is actively Q-switched by a
Pockels cell. A seed laser is used as frequency reference. The
injection seeding technique is used to achieve a single frequency mode
with a low-power continuous wave (CW) single frequency laser. The power
laser is conductively cooled via heat pipes. The transmitter assembly
will be operated in burst mode with 100 Hz PRF during 7 seconds (plus a
5 second warm-up time), in intervals of 28 seconds. There are two fully
redundant transmitters, each including two laser heads (Power Laser
Head and Reference Laser Head), and a TLE (Transmitter Laser
Electronics) module.

Receiver assembly: A combined
Mie and Rayleigh backscattering receiver is implemented. The receiver
assembly includes the transmit/receive switch (polarization-based), a
set of relay optics and diplexers for beam transport and laser
reference calibration, a blocking interference filter, the Mie and
Rayleigh receivers (spectrometers), and two DFU (Detection Frontend
Units).

• The Mie receiver consists of
a Fizeau spectrometer. The received backscatter signal produces a
linear fringe whose position is directly linked to the wind velocity.
The resolution of the Fizeau interferometer is 100 MHz (equivalent to
18 m/s). The wind value is determined by the fringe centroid position
to better than a tenth of the resolution. The backscattered signals are
detected by a thinned back-illuminated silicon CCD detector working in
an accumulation mode which allows photon counting. In the Mie channel,
the Doppler shift is estimated by measuring the displacement of
straight fringes produced by either a Fizeau or a two-wave
interferometer.

• The Raleigh receiver employs
a dual-filter (also referred to as double-edge) Fabry-Perot
interferometer (where the Doppler shift is estimated from the variation
of the signal transmitted through two filters located on both sides of
the broad Rayleigh spectrum) with a 2 GHz resolution and 5 GHz spacing.
It analyzes the wings of the Rayleigh spectrum with a CCD. The etalon
is split into two zones, which are imaged separately on the detector.
The wind velocity is proportional to the relative difference between
the intensities of the two etalons.

The optomechanical subsystem
of ALADIN uses a Cassegrain afocal telescope for both functions of
laser emission and backscatter reception. The optomechanical
architecture employs the monostatic observation concept: i.e., the
transmit and receive beams propagate through the same telescope. This
architecture allows to limit the instrument FOV: to ameliorate for
instance the daytime performance, and to relax the telescope and optics
stability requirements. TRO (Transmit-Receive Optics) is a
major subsystem of ALADIN, directing the laser pulses towards the
atmosphere, generating internal reference signals and feeding the
atmospheric return signal into the subsequent optical analyzers. 58)

The telescope design employs
isothermal and lightweight techniques based on SiC (Silicon Carbide)
type ceramic mirrors and structures. This concept provides the needed
optical quality and stability without a focusing or alignment
mechanism. Star trackers for attitude sensing are mounted on the
telescope structure to minimize the misalignment between the optical
axis and the telescope's line-of-sight.

The instrument
transmits raw source data consisting of the accumulated spectra from
the Mie receiver and the flux intensities from the Rayleigh receiver.
These data are provided for strips of 50 km length and a horizontal
resolution down to 3.5 km. In the vertical direction, many layers or
volume cells of the various altitude bins (nominally -1 km to 16.5 km
height for the Mie channel, and 0.5 km to 26.5 km for the Rayleigh
channel, but other scenarios can be uplinked in flight) are measured;
the instrument looks into a fixed direction (quasi perpendicular to the
flight path and 35º away from nadir) and provides a vertical wind
profile along the line of sight. In addition to these source data,
laser internal calibration and attitude data are transmitted, as well
as the receiver response calibration data.

The instrument performance considers
the SNR error for each channel at the indicated altitude range. In
addition, systematic bias errors are taken into account. When no ground
echo is retrieved, the measurement bias is not cancelled; the total
measurement error is slightly deteriorated. - For the Mie channel, the
LOS (Line-of-Sight) wind error is below the requirement of 0.6 m/s for
altitudes from 0 to 2 km in height. For the Rayleigh channel, the LOS
wind error is below the requirement (except a marginal performance
around 16 km).

Legend to Figure 33:
These are measurement performance estimates from the surface up to 30
km altitude. The red line indicates the observational requirements as
given by Table 6.

A programmable sequencer is
implemented for the detector permitting configuration changes with
regard to vertical altitude resolution and range coverage. The vertical
resolution can be varied from 250 m to 2 km or more. However, the
measurement accuracy is only obtained for the nominal vertical
resolution of 1 km. The altitude range is limited to 30 km. The
horizontal (along-track) onboard accumulation length can also be
changed between a distance of 1.0 km and 3.5 km.

In addition to
the horizontal line-of-sight (HLOS) velocity measurements, ALADIN is
able to provide information on cloud characteristics over the depth of
the atmosphere, as well as aerosol measurements in the troposphere.
These include:

Change of operational principle (change from burst mode to continuous mode for ALADIN laser):
The change of operational principle of the laser transmitter had minor
impact on the other sub-systems of ALADIN and on the platform. Exchange
of FPGA (Field Programmable Gate Arrays) in the TLE (Transmitter Laser
Electronics), the DEU (Detection Electronics Units) and the ALADIN
Control & Data Management unit (ACDM) as well as minor
modifications of the operation software and the ground processing
software are required (Ref. 55).

The laser transmitter is continuing
to be the greatest development challenge. Delays in the transmitter
program have resulted from two main problem areas, namely LIC
(Laser-Induced Contamination) caused by the interaction of the high
power UV beam with outgassing materials in the vicinity of optics, and
LID (Laser-Induced Damage) due to the fact that some of the optics are
near the “state of the art” in terms of surviving the high
fluences of the laser, particularly in the UV section. 59)

One of the most extensive test
programs for LID has been undertaken by DLR Stuttgart, on each of the
coating lots of all flight optics, along with a number of endurance
tests, in order to demonstrate sufficient LIDT margins for the duration
of the mission.

The first flight model of ALADIN
laser has been integrated and the second flight model integration is
being prepared. Once the lasers are fully characterized and delivered,
integration of ALADIN will resume.

Ground segment:

Spacecraft operations are performed
at ESOC (Darmstadt, Germany) using the Kiruna TT&C station. The
instrument data are received nominally by the ground station in
Svalbard (Spitzbergen). Additional X-band receiving stations (antenna
diameter as small as 2.4 m) can easily be added to provide a shorter
data delivery time.

The two primary components of the
Ground Segment are the FOS (Flight Operations Segment) and the PDS
(Payload Data Segment). The Aeolus ground segment at ESOC is scheduled
to use the latest version of the SCOS-2000 mission control system
(version 5).

For the complete mission duration
(launch up to the end of mission, when ground contact to the
spacecraft/payload is terminated), facilities and services will be
provided to the PDS (Payload Data Segment) located at ESA/ESRIN
(Frascati, Italy) for planning of scientific data acquisition. This
will include the uplink of instrument operation timelines as well as
the provision of scientific data downlink schedules based on the
predicted spacecraft orbit. The PDS will be responsible for measurement
data acquisition via the X-band station network, the preprocessing of
scientific data, and the scientific data archiving and distribution to
the Meteorological Centers and general scientific community. 60)61)

The FOCC (Flight Operations Control
Center) will operate from a dedicated control room at ESOC. Data
processing will be done at ESA/ESRIN, while wind profile retrieval will
be done by the ECMWF (European Centre for Medium-Range Weather
Forecasts), UK. Data ground processing to be completed within five
minutes after reception. 62)

Operational automation in the ground
segment: ADM-Aeolus opted to be one of the first missions to utilize
the mission automation systems developed as part of ESOC
infrastructure, namely MATIS (Mission Automation System) and SMF
(Services Management Framework). An initial simple automation approach
has been taken to allow Aeolus to set automation targets which would
have no impact on FOCC readiness for flight. Two simple initial targets
have been set:

- Automation of Control Center to TT&C station link configuration pre-pass and post-pass

- Automation of playback of the X-band HK data dumps.

A further automation phase will cover TM monitoring, analysis and reporting.

The PDS will be in charge of the
science data reception via X-band and of various processing, archiving
and product dissemination tasks. It will include the X-band acquisition
station located in Svalbard (Norway), the APF (Aeolus Processing
Facility ) located in Tromsø (Norway) for the processing and
dissemination of the Level 1B and Level 2A products, and the Level 2
Processing Facility (L2/Met PF) hosted by the ECMWF (European Centre
for Medium Range Weather Forecast) in Reading (UK).

Data products:

The primary data product of the
mission will be the Level 1B data set, comprising calibrated wind
velocity observations for both Mie and Rayleigh channels, with various
additional annotation parameters. With the continuous mode laser
operation, each observation profile will be constructed by the
averaging of N on-board accumulated measurements of P consecutive
pulses. Typical figures for N and P are, respectively, 30 and 20,
leading to an observation horizontal integration length of 90 km, with
less than 1% data gap between successive observations (instead of 50 km
in burst mode with 150 km data gap between observations). The different
values provided in Table 4 correspond to
the horizontal integration length that needs to be considered in order
to meet the wind velocity random error requirement.

The Level 1B products will be
globally delivered to a number of meteorological service centers within
3 hours after sensing (NRT service) and for selected regions within 30
minutes after sensing (QRT service, e.g. within 30 minutes).

Higher level products will include
information on clouds and aerosols optical properties (Level 2A), as
well as consolidated horizontal line-of-sight wind observations (Level
2B), after temperature/pressure corrections and scene classification of
the measurements within one observation. The assimilation of Level 2B
data in the ECMWF operational forecast model will provide the so-called
Aeolus assisted wind products (Level 2C).

Preparatory campaigns for the verification of the measurement principle

An A2D (Aladin Airborne Demonstrator)
instrument was developed by EADS Astrium SAS to demonstrate and
validate the capability of ALADIN. Installation and testing of the A2D
on ground was performed with first atmospheric signal in October 2005.
The two functional test-flights (Oct. 18 and 20, 2005) were performed
with signal from clear atmosphere, clouds and ground. The measurements
demonstrated that the aircraft integration and testing was successful.
These were probably the first flights of an airborne, direct-detection
Doppler wind lidar worldwide. 65)66)67)68)69)70)71)72)

Campaign

Location

Period

Duration

AGC-1 (Aeolus Ground Campaign-1)

DWD-MOL,
Lindenberg, Germany

October 2006

4 weeks (+1 week)

AGC-2

DWD Lindenberg, Germany

July 2007

2 weeks (+1 week)

Aeolus flight campaign 1

DLR Oberpfaffenhofen,
Lindenberg,

November 2007

15 days / 25 hours

Aeolus flight campaign 2

DWD Lindenberg, Germany and other European sites

Spring 2008

2 weeks (25 hours)

Table 8: Overview of the A2D validation campaigns on the Falcon aircraft

• In August 2009, DLR
performed a campaign on Germany’s highest mountain, the
Zugspitze. The clean mountain air was needed to provide the right
conditions to investigate what effects the atmosphere would have on the
return signal of the satellite's core instrument. The objective was to
accurately measure the spectrum of the backscattered laser light from a
lidar to further improve the measurements of wind speed. The
experiments were carried out by DLR at the Environmental Research
Station Schneefernerhaus observatory, which is located 2650 m above sea
level. The science measurements were done with the A2D. 73)

• In March 2010, a DLR team
conducted a flight campaign of 2 weeks in Iceland, performing a total
of six flights over Iceland, over the ocean between Iceland and
Greenland and over the Greenland glacier plateau. The aim of this
DLR-led campaign with A2D was to investigate details of the instrument
operations strategy and to refine the ADM-Aeolus data processors that
will provide the mission's wind products. 74)75)

Two different wind lidar instruments
– the A2D (ALADIN Airborne Demonstrator), and a reference wind
lidar operating at an infrared wavelength of two microns – were
operated onboard DLR's Falcon 20E aircraft, and both performed well
throughout the campaign.

• In May 2015, DLR is using
its Falcon research aircraft to test an aircraft-based version of the
wind measurement laser technology. From their temporary base in
Iceland, the researchers are flying over the ice sheets of southern
Greenland. As they do so, another proven wind lidar that was used over
Iceland to take volcanic ash measurements during the eruption of
Eyjafjallajökull in 2010 is being used as a reference and
comparison instrument on board the Falcon. The United States aerospace
agency, NASA, is also in Iceland, supporting the campaign with its own
research aircraft and measurement equipment. 76)

- Windiest place on Earth: Europe's
weather systems are formed in the arctic polar region around Iceland
and Greenland. Small anomalies that occur where cold air masses from
the polar regions meet warmer air masses can lead to the development of
weather systems. The Icelandic depressions here are well known. In
addition, the polar region of Greenland is of particular interest in
climate research because of the rising temperatures in the Arctic and
the associated retreat of polar ice sheets. "In the current research
flight campaign, we are calibrating the new wind lidar above the
extensive ice fields of Greenland – testing our algorithms in the
process – to make sure that, later on, everything runs smoothly
in space," says Oliver Reitebuch from the DLR Institute of Atmospheric
Physics. In particular, the southern tip of Greenland – the
windiest place in the world – is the perfect testing ground for
the new wind measurement technology, as it is especially challenging,
with pronounced tip jets and strong jet streams.

- NASA and DLR
joint flights: "We are conducting several flights per day above the
permanent ice of Greenland and, in doing so, are acquiring comparative
data from a summit station operated by our US research colleagues at an
altitude of 3200 m," says DLR test pilot Philipp Weber. After taking
off from Iceland, the Falcon crew makes a refuelling stop at
Kangerlussuaq in Greenland and then spends two hours crisscrossing
Greenland. In total, some 10 test flights above Greenland are planned,
which will mostly take place in coordination with the NASA DC-8
research aircraft. The data from the NASA DC-8 and the DLR Falcon will
then be compared. Two lidar instruments are being used on board the
NASA DC-8, in addition to measurement probes that are ejected from the
aircraft via a chute.

- Scattered light makes wind fields
visible: At present, the major wind fields over the oceans are still
detected optically by weather satellites tracking cloud movements, or
measured indirectly using radar signals reflected by the wave motion on
the surface of oceans. "The wind lidar measurements will enable the
project team to directly measure wind speeds from ground level up to an
altitude of 20 km with significantly greater accuracy. Depending on the
altitude, the project can achieve a resolution of between 500 to 1000 m
while doing so," explains Reitebuch. "With the Doppler lidar –a
laser pulse sequence is emitted into a wind field at a precisely
defined wavelength. Depending on the movement of the wind field, the
light is reflected back with a very small change in wavelength. From
this, the team can determine the wind speed," continues Reitebuch.
Using this technology, the DLR researchers will be capable of
accurately determining changes as small as one ten billionths of a
wavelength.

- Small anomalies with large effects
on the weather: In addition to testing the wind lidar above Greenland,
the DLR atmospheric researchers are acquiring data on the formation and
development of Icelandic depressions. The researchers hope to better
understand how low-pressure systems arise from small anomalies over
Iceland, Greenland and the North Atlantic in a short time. "From
Iceland, measurements can be performed in the strong jet streams over
the North Atlantic. Detailed knowledge of the wind distribution is
particularly important because a lack of wind data very quickly leads
to errors in weather forecasting models," says Reitebuch. "These errors
affect the accurate forecasting of the development of low-pressure
systems, which often move towards Europe and, due to their high winds
and heavy rainfall, have a significant effect on our daily lives."

- The DLR ADM (Atmospheric Dynamics
Mission) research flight campaign over Iceland and Greenland is a DLR
contribution to the ESA ADM-Aeolus mission. Involved in this mission
are the DLR Institute of Atmospheric Physics, DLR Flight Experiments,
ESA and the University of Leeds, which has a wind lidar installed at
the summit station in Greenland for the mission to perform comparison
measurements from the ground. This mission is being carried out in
cooperation with NASA. For the first time in the world, four wind lidar
instruments on two aircraft are being used at the same time.

Figure 36: Photo of the DLR
Falcon (foreground) and the NASA DC-8 aircraft prior to the joint
research flight campaign from Iceland (image credit: DLR)

Design and setup of the ALADIN airborne demonstrator:

The core of the A2D is based on the
ALADIN receiver and transmitter from the pre-development program of ESA
and is therefore representative of the actual satellite instrument. The
optical receiver of the A2D was space qualified with respect to its
thermal vacuum and vibration environment during the pre-development
phase.

The A2D is a nonscanning lidar as
the satellite instrument. Thus, only one LOS component of the
three-dimensional wind vector is measured in contrast to most other
direct-detection wind lidars, which are equipped with a scanning
device. The LOS wind is measured perpendicular to the aircraft roll
axis, with an off-nadir angle of 208. The A2D is designed to be
operated on the DLR Falcon 20 aircraft, a twin-engine jet with a
pressurized cabin allowing a maximum payload of 1.1 ton, a flight
altitude of up to 12 km, and range of up to 3700 km.

The installation of the A2D inside the Falcon aircraft is shown in Figure 37
with the telescope, the mechanical aircraft frame, and the thermal hood
of the receiver system. The mechanical frame holding the telescope,
receiver, and laser is mounted via vibration-damping shock mounts to
the seat rails of the aircraft. The mechanical frame of the 10.6
µm heterodyne wind infrared Doppler lidar, which has proven its
aircraft vibration-damping behavior needed for coherent detection, was
adapted to hold the A2D laser, optical receiver, and telescope.

The laser beam is directed toward
the atmosphere via a window in the bottom fuselage of the aircraft
cabin. The electronic units operating the A2D are installed in 19 inch
aircraft racks and are controlled by two operators. The total volume of
the system is 3 m3, the mass is 550 kg, and the mean power
consumption is 2.5 kW. Finite element simulations were performed to
minimize the overall weight, providing high stiffness for the transmit
and receive optical path, and to prove airworthiness.

Optical design
overview: The narrowband single-frequency laser pulses at 354.89 nm
vacuum wavelength are generated by an Nd:YAG laser. The circularly
polarized laser pulses are transmitted via three reflecting mirrors
through the aircraft window (or one reflecting mirror in case of ground
operation) toward the atmosphere. The last reflecting mirror is placed
on the telescope optical axis and thus a coaxial transmit–receive
system is obtained.

The backscattered photons from the
atmosphere are collected by a 20 cm aperture Cassegrain telescope and
directed to the optical receiver via an optical relay with two lenses
and two mirrors. After passing the front optic with field and aperture
stop, the light is directed toward the two spectrometers. The Rayleigh
spectrometer uses the double-edge technique with a sequential
Fabry–Perot interferometer, whereas the Mie spectrometer is based
on a Fizeau interferometer. For both the Rayleigh and the Mie
spectrometer, an ACCD (Accumulation CCD) detector is used, and the
electronic signal is digitized after preamplification. The sequential
implementation of the Fabry–Perot interferometer and the ACCD are
patented by Astrium.

The optical beam path with about 60
optical elements and the alignment sensitivities were studied in detail
with an optical ray-tracing model. The principle layout of the A2D
optical design is shown in Figure 38. The main instrument parameters for the satellite ALADIN and the A2D are summarized in Table 9.

Table 9: Specifications of the satellite ALADIN and measured performance of the A2D

Development status of the spacecraft and ALADIN

• August 16, 2018: Measuring 4.5 m across, this relatively small antenna in Australia, dubbed NNO-2, will be the first to hear from the soon-to-be-launched Aeolus satellite, the first ever to measure winds on Earth from Space. 77)

- Aeolus’ first steps after
separation will include the automatic unfolding of its solar
‘wings’ and turning its antenna to face Earth to start
sending signals. Only then will teams on the ground be able to get any
sign from the satellite that all is well.

- Since 2015, NNO-2 has been
pointing to space, listening for signals from rockets and newly
launched satellites and transmitting instructions and commands to them
from engineers on Earth.

• August 9, 2018: As
preparations for the launch of ESA’s latest Earth Explorer
continue on track, the team at Europe’s Spaceport in French
Guiana has bid farewell to the Aeolus satellite as it was sealed from
view in its Vega rocket fairing. Liftoff is set for 21 August at 21:20
GMT (23:20 CEST). 78)

- Since its arrival at the launch site in early July, Aeolus has been thoroughly tested and fuelled with hydrazine.

- Like all of ESA’s Earth
Explorer missions, Aeolus will fill a gap in our knowledge of how our
planet works and show how novel technology can be used to observe Earth
from space.

Figure 39:
Encapsulation - Aeolus carries one of the most sophisticated
instruments ever to be put into orbit. The first of its kind, the
Aladin instrument includes revolutionary laser technology to generate
pulses of ultraviolet light that are beamed down into the atmosphere to
profile the world’s winds – a completely new approach to
measuring the wind from space (image credit: ESA/CNES/Arianespace)

• August 2, 2018: With liftoff
less than three weeks away, ESA’s Aeolus satellite has been
fuelled and is almost ready to be sealed within its Vega rocket
fairing. 79)

Figure 40: With liftoff less than
three weeks away, ESA’s Aeolus satellite has been fuelled and is
almost ready to be sealed within its Vega rocket fairing. Getting a
satellite ready to be launched involves a long list of jobs, some of
which are trickier than others. Since hydrazine is extremely toxic,
only specialists dressed in bulky astronaut-like suits remained in the
cleanroom for the duration of the activity (image credit:
ESA/CNES/Arianespace)

• July 24, 2018: The launch of
Aeolus — ESA’s mission to map Earth’s wind in
realtime — is getting close, with the satellite due for lift-off
on 21 August from Europe’s Spaceport in Kourou, French Guiana.
With the wind in their sails, mission teams are busily preparing this
unique satellite for its upcoming journey. 80)

- Aeolus will carry a sophisticated
atmospheric laser Doppler instrument, dubbed ALADIN. Combining two
powerful lasers, a large telescope and extremely sensitive receivers,
it is one of the most advanced instruments ever put into orbit.

- Currently one
of the biggest challenges in making accurate weather predictions is
gathering enough information about Earth’s wind. Aeolus will be
the first-ever satellite to directly measure winds from space, at all
altitudes, from Earth's surface through the troposphere and up 30 km to
the stratosphere — providing information that will significantly
improve the quality of weather forecasts.

- Paolo Ferri, Head of Mission
Operations at ESA adds, “The Aeolus mission will be a wonderful
addition to our fleet of satellites that continually observe Earth
bringing us incredible insights into our planet, in particular into the
complex world of atmospheric dynamics and climate processes —
systems that not only affect our everyday lives but also have huge
consequences for our future.”

Figure 41:
Earth’s wind patterns: The movement of air constitutes the
general circulation of the atmosphere, transporting heat away from
equatorial regions towards the poles, and returning cooler air to the
tropics. Atmospheric circulation in each hemisphere consists of three
cells - the Hadley, Ferrel and polar cells. High-speed wind fields,
known as ‘jets’, are associated with large temperature
differences (image credit: ESA/AOES Medialab)

• July 10, 2018: With the
campaign to launch ESA’s Aeolus wind satellite on 21 August well
underway, the satellite’s telescope has been opened and inspected
to make sure it is perfectly clean and shiny. 81)

- While Aeolus’ novel laser
technology is arguably the sexy part of the instrument, its telescope,
which measures around 1.5 m across, is pretty dominant and equally
important. It is used to collect backscattered light from the
atmosphere and direct it to the receiver. In short, the laser system
generates a series of short pulses of ultraviolet light which are
beamed down into the atmosphere. The telescope collects the light
backscattered from particles of gas and dust in the atmosphere. The
time between sending the light pulse and receiving the signal back
determines the distance to the ‘scatterers’ and therefore
the altitude above Earth. As the scattering particles are moving in the
wind, the wavelength of the scattered light is shifted by a small
amount as a function of speed. The Doppler wind lidar measures this
change so that the velocity of the wind can be determined.

- It is clearly important to make
sure that the instrument is absolutely spotless, so engineers at the
launch site in Kourou have first turned to the telescope and given it a
close inspection.

Figure 42: Aeolus shiny telescope (image credit: ESA)

• July 6, 2018: Having set
sail from France on 15 June - Global Wind Day, ESA’s Aeolus wind
satellite has arrived safe and sound at the launch site in French
Guiana. - While almost all satellites travel by aircraft, Aeolus’
journey was rather different – it travelled all the way across
the Atlantic from Saint Nazare, western France to the Port of Cayenne,
French Guiana by ship. 82)

- Aeolus carries
one of the most sophisticated instruments ever to be put into orbit. A
12-day journey was undertaken to avoid potential damage caused by air
re-pressurization during descent had the satellite travelled by air
– a quicker but decidedly riskier option.

- Upon its long-awaited arrival, the
team unloaded Aeolus and its support equipment. The containers were
then carefully positioned on a truck to be transported to the launch
site about 60 km away, where the satellite container was moved into the
airlock, to stabilize after its long journey.

- The satellite was then removed
from its container, placed on its integration trolley for testing and
connected to its electrical support equipment. Initial checks indicate
that Aeolus has withstood its journey from France in good condition.

- ESA’s Aeolus project
manager, Anders Elfving, said, “We are obviously all extremely
pleased that Aeolus has now arrived at the launch site. An awful lot of
work and planning went into making sure it arrived safe and sound
– now it’s full steam ahead for preparing the satellite for
liftoff on 21 August.”

- A range of checks will be carried
out on the satellite in the cleanroom before the scheduled liftoff on a
Vega rocket on 21 August at 21:20 GMT (23:20 CEST) from Europe’s
spaceport near Kourou.

• June 15, 2018: Today is
Global Wind Day, which couldn’t be more apt for ESA’s
Aeolus wind satellite to begin its voyage to the launch site in French
Guiana. And, while almost all satellites journey by aircraft, Aeolus is
different, it’s going by ship. 83)

- Since the ALADIN instrument is
sensitive to pressure change, ESA and Airbus Defence and Space
engineers decided that the safest way for it to journey from France,
where it has been going through testing, to French Guiana would be by
ship.

- Denny Wernham, ESA’s Aeolus
instrument expert, explains, “Going by ship may seem a little
strange, after all it will take around 12 days to get there instead of
a matter of hours, but if, for whatever reason, the aircraft had to
descend rapidly and there was a sudden increase in air pressure,
Aeolus’ instrument could be damaged.

- “It was designed, of course,
to allow for the pressure drop during launch ascent so that it could be
taken into orbit, but not for a fast descent. So basically, once
it’s up, it’s up.

- “So, today we see our
beloved satellite and all of its support equipment being loaded onto a
ship in Saint Nazaire in western France and set forth across the
Atlantic. And, indeed, it is kind of ironic: our high-tech wind
satellite is travelling by a means that many years ago relied on the
wind.”

- Aeolus has, without doubt, been a
challenging satellite mission to develop. Nevertheless, this
long-awaited mission is now set to not only improve our understanding
of how the atmosphere works and contribute to climate change research,
but will also help to predict extreme events such as hurricanes. It
will also help to better understand and model large-scale wind patterns
driving weather such as El Niño.

Figure 44: The ship, painted with
‘Airbus onboard’, waiting for ESA’s Aeolus satellite
to arrive. The vessel will carry Aeolus from Saint Nazaire in western
France to the launch site in French Guiana. Liftoff is scheduled for 21
August 2018 (image credit: ESA–G. Labruyere) 84)

• June 5, 2018: Like all of
the Earth Explorers, Aeolus was built to show how cutting-edge space
technology can shed new light on the intricate workings of our planet.
This pioneering satellite uses powerful laser technology that probes
the lowermost 30 km of our atmosphere to yield vertical profiles of the
wind as well as information on aerosols and clouds. This will not only
improve our understanding of how the atmosphere works and contribute to
climate change research, but will also help to predict extreme events
such as hurricanes and El Niño. 85)

Figure 45: Before ESA’s
Aeolus satellite is packed up and shipped to French Guiana for liftoff
in August, media representatives had the chance to see this wind
measuring Earth Explorer satellite standing proud in the Airbus Defence
and Space cleanroom in Toulouse, France (image credit: ESA, M.
Pedoussaut)

• February 7, 2018:
ESA’s Aeolus satellite has been particularly tricky to build. One
of the main stumbling blocks has been getting its lasers to work in a
vacuum, but recent tests on the satellite show that the vacuum or
temperature of space won’t get in the way of Aeolus measuring
Earth’s winds. 86)

- The ALADIN instrument shoots
pulses of ultraviolet light down into the atmosphere and measures the
backscattered signals from molecules and aerosols to profile the
world’s winds.

- “This will be the first time
that we will be able to directly measure profiles of the global wind
field from space in cloud-free conditions. It has been a major
challenge for us all – our ESA engineers, industry, our Member
States – to overcome many technical and programmatic challenges.
I am grateful to everyone for having gone through this and for having
trust in ESA to finally make it happen. We are now very close to seeing
the fruits of a long endeavor,” said Josef Aschbacher,
ESA’s Director of Earth Observation Programs.

- These vertical slices through the
atmosphere, along with information on aerosols and clouds, will advance
our knowledge of atmospheric dynamics and contribute to climate
research.

- Since Aeolus will deliver
measurements almost in realtime, it is also set to provide much-needed
information to improve daily weather forecasts.

- The
satellite’s novel technology was under development for some
years, but issues with the laser component of the instrument and with
the optics, which have to survive exposure to the high-intensity laser
pulses, were eventually resolved, and in 2016 the instrument was
finally ready.

Figure 46: Laser reading: The
image indicates that the laser carried on ESA’s Aeolus satellite
works well in a vacuum. ESA’s Aeolus satellite spent nearly two
months in a thermal–vacuum chamber to make sure that its novel
instrument will work as it should in space. Aeolus carries one of the
most sophisticated instruments ever to be put into orbit: Aladin, with
two powerful lasers, a large telescope and very sensitive receivers. It
will be the first such satellite mission to measure Earth’s winds
from space. It actually carries two laser transmitters just in case one
fails (image credit: ESA)

- ALADIN was
then added to the satellite in the UK, after which the assembly was
moved to France where it was shaken to simulate the rigors of liftoff.

- The last round of tests was
carried out in CSL (Centre Spatial de Liège), Belgium, and
involved putting the satellite in a thermal–vacuum chamber for
almost two months (Figure 47).

- Once the satellite was safely
inside, the air was pumped out and the chamber cooled by liquid
nitrogen to simulate the environment of space – and then Aeolus
was put through its paces.

- ESA’s Aeolus project
manager, Anders Elfving, said, “The test was exceptionally
complex, not only because it was a tight fit with the satellite filling
up most of the space in the chamber, but also because we had to make
sure that the whole instrument’s performance is tip-top. It was
an extremely technical and delicate undertaking that included firing
ALADIN’s lasers at full power. The satellite as a whole came
through with flying colors, and we are particularly pleased that the
two laser transmitters performed brilliantly.”

- With this milestone behind it,
Aeolus has now been returned to France where it will have a few final
tests before being shipped across the Atlantic to Europe’s
Spaceport in French Guiana for launch on a Vega rocket in the autumn.

- ALADIN was built by Airbus SAS in
Toulouse, France, the satellite by Airbus Ltd. in Stevenage, UK, and
the laser transmitters by Leonardo SpA in Florence and Pomezia, Italy.

• November 2, 2017: With
liftoff on the horizon, ESA’s Aeolus satellite is going through
its last round of tests to make sure that this complex mission will
work in orbit. Over the next month, it is sitting in a large chamber
that has had all the air sucked out to simulate the vacuum of space. 87)

- Aeolus carries one of the most
sophisticated instruments ever to be put into orbit: Aladin, which
includes two powerful lasers, a large telescope and very sensitive
receivers. The laser generates ultraviolet light that is beamed down
into the atmosphere to profile the world’s winds – a
completely new approach to measuring the wind from space.

- These vertical slices through the
atmosphere, along with information it gathers on aerosols and clouds,
will improve our understanding of atmospheric dynamics and contribute
to climate research. As well as advancing science, Aeolus will play an
important role in improving weather forecasts.

- With these difficulties in the
past, the satellite is now undergoing final testing in Belgium before
it is shipped to French Guiana for liftoff, which is scheduled for the
middle of next year.

- After having spent this spring at
Airbus Defence and Space in Toulouse, France, where it was checked that
it could withstand the vibration and noise liftoff and its ride into
space, Aeolus has been at the Centre Spatial de Liège since May.
- Here, it has just been enclosed in the thermal–vacuum chamber
for the next 30 days or so.

- With the satellite safely inside, the chamber door was closed a few days ago and the air was pumped out to create a vacuum.

- Denny Wernham, ESA’s Aladin
instrument manager, said, “It takes some time for the air and
outgassing from the satellite to be pumped out of the chamber, but
Aeolus finally faced ‘hard vacuum’ on 31 October.

- “Tests are scheduled to run
continuously over the next 33 days. We are particularly keen to see how
well the laser transmits its pulses of ultraviolet light and the
alignment of the instrument in this environment.

- Once these tests are done, the
satellite will be transported back to Toulouse for final checks before
being shipped across the Atlantic to Europe’s Spaceport in French
Guiana for launch on a Vega rocket.

• January 30, 2017: The road
to realizing ESA’s Aeolus mission may have been long and bumpy,
but developing novel space technology is, by its very nature,
challenging. With the satellite now equipped with its revolutionary
instrument, the path ahead is much smoother as it heads to France to
begin the last round of tests before being shipped to the launch site
at the end of the year. 88)

- Aeolus carries one of the most
sophisticated instruments ever to be put into orbit: ALADIN, with two
powerful lasers, a large telescope and very sensitive receivers. It
shoots pulses of ultraviolet light down into the atmosphere to profile
the world’s winds. This is a completely new approach to measuring
the wind from space, which usually involves tracking cloud movement,
measuring the roughness of the sea surface or inferring wind from
temperature readings.

Figure 48: Now that Aeolus is
equipped with its ALADIN instrument, it is ready to be moved from
Airbus Defence and Space in the UK to their facilities in Toulouse,
France. There it will start the last round of tests before being
shipped to the launch site (image credit: Airbus DS)

Legend to Figure 49:
The ADM-Aeolus mission will not only advance our understanding of
atmospheric dynamics, but will also provide much-needed information to
improve weather forecasts. The satellite carries the first wind lidar
in space, which can probe the lowermost 30 km of the atmosphere to
provide profiles of wind, aerosols and clouds along the
satellite’s orbital path. The laser system emits short powerful
pulses of ultraviolet light down into the atmosphere. The telescope
collects the light that is backscattered from air molecules, particles
of dust and droplets of water. The receiver analyses the Doppler shift
of the backscattered signal to determine the speed and direction of the
wind at various altitudes below the satellite. These near-realtime
observations will improve the accuracy of numerical weather and climate
prediction and advance our understanding of atmospheric dynamics and
processes relevant to climate variability.

- Aeolus has been built mainly to
advance our understanding of Earth. These vertical slices through the
atmosphere, along with information on aerosols and clouds, will advance
our knowledge of atmospheric dynamics and contribute to climate
research. - However, Aeolus also has a very important practical role to
play because its measurements will be delivered rapidly, improving
weather forecasts. After its long development, ALADIN was finally ready
to join the satellite at Airbus Defence and Space in Stevenage in the
UK in August last year.

Figure 50:
Standing proud: ESA’s Aeolus satellite in the cleanroom at Airbus
Defence and Space in Stevenage, UK. During the last half of 2016 the UK
team with support of their colleagues from Toulouse in France worked
tirelessly to integrate the ALADIN instrument into the satellite, to
check that all is aligned and that the complete satellite is working
flawlessly. As the sole measuring instrument on the Aeolus satellite,
ALADIN comprises two powerful lasers, a large telescope and very
sensitive receivers. It is designed to probe the lowermost 30 km of the
atmosphere to provide profiles of wind, aerosols and clouds along the
satellite’s orbital path (image credit: Airbus DS)

- With the satellite now complete,
it is time move it to Toulouse where it will be tested to make sure
that it can withstand the vibration and noise of liftoff. — After
this, ADM-Aeolus will go to Liege in Belgium to be checked in a
thermal–vacuum chamber.

• August 2, 2016: After many
years in development, ALADIN – the Doppler wind lidar to be
carried on the Aeolus satellite – is ready to be shipped from
Toulouse, France, to the UK to be installed on the satellite in
preparation for liftoff by the end of 2017. Aeolus will be the first
satellite mission to probe the wind globally. These vertical slices
through the atmosphere, along with information on aerosols and clouds,
will advance our knowledge of atmospheric dynamics and contribute to
climate research. 89)

- Its state-of-the art ALADIN
instrument incorporates two powerful lasers, a large telescope and very
sensitive receivers. The laser generates ultraviolet light that is
beamed towards Earth. This light bounces off air molecules and small
particles such as dust, ice and droplets of water in the atmosphere.
The fraction of light that is scattered back towards the satellite is
collected by ALADIN’s telescope and measured.

Figure 51: Photo of the ALADIN
telescope and instrumentation at Airbus DS in Toulouse to be shipped to
Airbus DS UK for installation into the ADM/Aeolus spacecraft (image
credit: Airbus DS)

• 2016: The ALADIN instrument
is fully integrated and both laser transmitters are aligned for optimal
performance. The In-situ Cleaning Subsystem was tested together with
the latest satellite flight software. The satellite platform was
finalized and checked out in preparation for mating with the ALADIN
PLM. The Payload Data Ground Segment facilities are being prepared and
undergoing tests. The Flight Operations Segment facilities are also
being readied. 90)

• April
22. 2015: A lot of time has gone into developing the technology
involved and testing both lasers. Despite numerous setbacks, in
particular issues associated with them working properly in a vacuum,
engineers at Selex-ES in Italy persevered. Thanks to their dedication
and ingenuity, a major milestone for the mission has been achieved.
Both lasers have now been delivered to Airbus Defence and Space in
Toulouse, France, ready to be integrated into the rest of ALADIN. 91)

- Despite a number of setbacks, this
cutting-edge piece of technology is now ready to be integrated into the
rest of the satellite’s instrument – a Doppler wind lidar
called ALADIN. — ADM-Aeolus will provide profiles of the
world’s winds as well as information on aerosols and clouds.
These profiles will not only advance our understanding of atmospheric
dynamics, but will also offer much-needed information to improve
weather forecasts.

- Thanks to these collective
efforts, the project can now focus on the instrument and satellite
integration and testing. This means a launch of ADM-Aeolus spacecraft
can be done in 2016.

Figure 52: Photo of the
ADM-Aeolus second ALADIN laser prior to closure showing the complexity
of the 80 optical components contained within a relatively small space
of 45 x 34 x 20 cm and a mass of ~30 kg (image credit: Selex-ES)

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net)